The valve timing control device described above may be provided with a so-called intermediate lock mechanism that fixes the relative rotational phase of the driving side rotating body and the driven side rotating body to an intermediate phase appropriate for the starting of the internal combustion engine when the internal combustion engine is started. For example, the intermediate lock mechanism quickly sets the relative rotational phase of the driving side rotating body and the driven side rotating body to a predetermined intermediate phase when the internal combustion engine is stopped and allows lock members disposed in one of the rotating bodies to project into engagement grooves disposed in the other rotating body so that the relative rotational phase of both of the rotating bodies is fixed.
This state is maintained during the starting of the internal combustion engine. After the internal combustion engine is started, an intermediate lock state is maintained until hydraulic pressure control for the relative rotational phase is reliably performed with a required hydraulic pressure raised. According to the technique disclosed in JP2004-257313A, for example, the engagement grooves have a stepped shape so that a transition to the intermediate lock state can be expedited when, for example, the internal combustion engine is stopped. In this case, the two lock members can be sequentially engaged with the respective engagement grooves and the transition to the intermediate lock state can be expedited.
In the device according to the related art described above, the lock members are configured to be always spring-biased toward the engagement grooves and the escape of the lock members from the engagement grooves based on a centrifugal force resulting from the rotation of the driving side rotating body and the driven side rotating body during the starting of the internal combustion engine is configured not to occur. In a case where a driver suddenly increases the rotation speed after the starting of the internal combustion engine, however, the lock is unexpectedly released due to the generation of a centrifugal force exceeding the biasing force of the spring or an increase in the vibration of the internal combustion engine, which causes the operation state of the internal combustion engine to be disturbed.
In this case, the problem can be addressed when phase retention control is performed in the unlock state. In some cases, however, the required hydraulic pressure has yet to be achieved immediately after the starting of the internal combustion engine or intermediate retention control is not performed well with the viscosity of the hydraulic oil high during, for example, a cold start. In this case, the relative rotational phase is subjected to the lack of uniformity. This phase irregularity is particularly significant in an intermediate lock-type device. As a result, proper exhaust, fuel economy, and output performances are not achieved unless precise advance and retard phase change operations are performed.